When extruding polyvinyl chloride (PVC) pipe, an adjustable sizing tube is used to guide, create and maintain the final outside dimensional configuration of the extruded pipe material before the pipe material has completely hardened. The sizing tube is located within a vacuum-calibration tank (VCT) which is located downstream from an extruder and respective extrusion die. The sizing tube can be either fixed or adjustable. However, the fixed sizing tube is made from a rigid-walled tube and cannot be adjusted. Both types of sizing tubes are characterized by circumferential slotted channel configurations.
As shown in FIGS. 1A and 1B, a prior art adjustable sizing tube 10 has various slotted channel 14 configurations cut circumferentially throughout the length of the sizing tube 10. The slotted channels 14 allow a negative vacuum created in the VCT's chamber to “pull” the extruded pipe material against the inner wall of the sizing tube 10 to maintain the required outer diameter dimensional characteristics of the extruded pipe.
As shown in FIG. 1B, the prior art adjustable sizing tube 10 has a single slot 18 cut the entire length of the tube 10 which allows for minor reductions to the diameter of the sizing tube 10.